Method and apparatus to determine the direction to a transponder in a modulated backscatter communication system

ABSTRACT

Methods and systems for determining the direction to a transponder are disclosed. The methods and systems include transmitting a first signal to an area where communications with a transponder is desired; producing a second signal desired from the first signal; receiving the second signal via a first and second antenna forming a difference signal from the second signal received via the first and second antennas; forming a third signal by adding the second signal received via the first antenna and the second signal received via the third antenna; delaying the difference signal; and comparing a fist polarity of the delayed difference signal with a second polarity of the third signal.

REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/209,352, filed Jun. 5, 2000, whose disclosure ishereby incorporated by reference in its entirety into the presentdisclosure.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to radio frequency identification(RFID) readers and transponders, and more particularly, to an RFIDreader that can determine the direction of movement of a transponderrelative to the reader in a modulated backscatter communication system.

[0004] 2. Description of Related Art

[0005] In the automatic identification industry, the use of RFIDtransponders has grown in prominence as a way to track data regarding anobject to which the RFID transponder is affixed. An RFID transpondergenerally includes a semiconductor memory in which digital informationmay be stored. A known technique for communicating with RFIDtransponders is referred to as “backscatter modulation,” whereby theRFID transponders transmit stored data by modulating their antennamatching impedance to reflect varying amounts of an electromagneticfield generated by the RFID reader. An advantage of this communicationtechnique is that the RFID transponders can operate independently of thefrequency of the energizing electromagnetic field, and as a result, thereader may operate at multiple frequencies so as to avoid radiofrequency (RF) interference, such as using frequency hopping spreadspectrum modulation techniques. The RFID transponders may extract theirpower from the energizing electromagnetic field, thereby eliminating theneed for a separate power source.

[0006] In many applications, it would be desirable for the RFID readerto derive location and direction information from the RFID transponderin addition to the stored data.

[0007] Typically, the determination of relative location between theRFID transponder and reader has been controlled by a combination ofsystem sensitivity and antenna patterns. For example, an antenna ofappropriate gain pattern can be used to provide coverage of a singlelane of traffic so that an interrogator can establish communicationswith RFID transponders on vehicles in that lane and no other. This andsimilar techniques have been successfully used in systems where at mostone object carrying a transponder can be physically located in thecoverage area of the reader antenna. An ambiguity arises, however, whenseveral transponders are located in the coverage area of the readerantenna. For such situations, it is necessary to physically determinewhich transponder has established communications with a reader, oralternatively, what the location of the transponder is with reference tothe location and orientation of the reader.

[0008] There are many known techniques for determining the directionfrom a single location to a transponder using an array of antennas andmeasuring signal strength or arrival direction of the wave transmittedby the transponder. Hane (U.S. Pat. No. 4,728,955) describes one suchtechnique to locate a modulated backscatter transponder using an antennaarray. The transponder produces a single sideband suppressed carriermodulated backscatter signal that contains modulation of a subcarrier.The direction of arrival is determined by measuring the phase of thesignal in each of several antennas and calculating the direction ofarrival based on the measured phases. A significant drawback of thistechnique is that it is complex, cumbersome, and relies on carefullymaintaining linearity in amplifiers and detectors. Moreover, the Hanetechnique includes measuring phase angles and using a computer tocalculate the direction to the transponder, and is thus ill suited toreceivers using limiting amplifiers, such as those shown by Koelleet.al. (U.S. Pat. No. 4,739,328).

[0009] Limiting amplifiers eliminate detailed phase information thatwould be necessary to accurately determine location using the Hanetechnique. The output of a limiting amplifier only provides informationof whether the signal is from 0±90° from the reference phase (i.e., inphase) or is from 180±90° (i.e., out of phase) from the reference phase.All detailed phase information is therefore lost in a limitingamplifier.

[0010] More specifically, the Hane technique is ill suited for modulatedbackscatter systems that use direct modulation of the microwave carrieras opposed to the single sideband technique. That is because the outputof the mixers in Hane will experience “quadrature nulls” withtransponder position for transponders that do not produce a singlesideband suppressed carrier signal. Koelle et. al. eliminate the“quadrature null” effect by using a multi-channel receiver and limitingamplifiers in the reader to communicate with the transponders. Thus, theHane technique would not provide direction information for a transponderof the type disclosed by Koelle et al. even if the mixers of Hane arereplaced with a multi-channel homodyne receiver of Koelle et al. in viewof the use of limiting amplifiers.

[0011] An alternative direction finding system was disclosed by Koelleet. al. (U.S. Pat. No. 5,510,795). According to Koelle et. al., thedirection finding system measures whether a transponder is moving towardor away from the reader. If the transponder is moving past a reader, thedirection finding system will provide an indication of when the movementof the transponder is zero in the direction of the reader. Unless thepath of the transponder is restricted (e.g., mounted on an object onrails), the system cannot be used to determine the direction to thetransponder. Likewise, the system cannot be used to determine thedirection to the transponder if the reader antenna is rotated in asearching mode since the distance between a transponder and the readerdoes not change in that case.

[0012] It is also possible to determine the location of a transponderusing a bi-static homodyne radio system where the transmit antennas andreceive antennas are separated by a considerable distance as compared tothe microwave wavelength. Such a geometry is described in R. J. King,Microwave Homodyne Systems, pp. 206-216 (1978). Communication withtransponders takes place where the gain patterns of the transmit andreceive antenna of the reader system intersect. That intersectiondefines the area in space where communications with the transponder arepossible. Such a system is not useful for a compact, handheld reader norfor determining the location of a particular transponder if severaltransponders are located in the communication zone.

[0013] Other direction finding techniques are used for determining thedirection to a conventional radar target. A method known as“simultaneous lobing” or “monopulse” is described in M. I. Skolnik,Introduction to Radar Systems, pp. 175-184 (McGraw-Hill 1962). Accordingto this method, collocated or closely spaced antennas forming sum anddifference beams use phase and/or amplitude detectors to determineprecisely when a radar beam is swept across a remote target. A drawbackof this method is that it cannot be used to determine the direction to abackscatter transponder when it is in the vicinity of other scatteringobjects which produce signals stronger than those of the transponder.Additionally, typical antenna patterns of reader antennas are ofrelatively low gain (e.g., 6 to 15 dBi) because of the necessity ofproviding the required coverage area. If low gain antenna elements areused, the offset feed technique to produce two beams to form sum anddifference beams results with a difference beam having very small gain,which is not useable for RFID applications. Thus, the normal directionfinding techniques used in conventional radar systems are not applicableto modulated backscatter systems.

[0014] Accordingly, it would be desirable to determine the directionfrom a reader to a transponder in a backscatter communications systemwhere the amplification of signals is performed using limitingamplifiers.

SUMMARY AND OBJECTS OF THE INVENTION

[0015] The present invention discloses methods and systems to determinethe direction to a transponder in a modulated backscatter communicationsystem. More particularly, such systems and methods make use of at leasttwo antennas to receive a scattered or modulated signal from atransponder, such as a RF ID backscatter tag. An additional antenna isused as a reference from which the direction of the transponder isdetermined and is also used to transmit the signal to the transponderwhich then becomes modulated or scattered.

[0016] The signal received on the two antennas is used to form adifference signal. Since receiving the signal through the two antennascould have the effect of producing two different versions (i.e.,different phases) of the signal, subtracting the outputs of the antennaswill produce a difference signal that may not have a value of zero. Thatdifference signal is delayed by 90 degrees. The polarity of the delayedsignal is compared with the polarity of the scattered or modulatedsignal as received by the reference antenna to determine the directionof the transponder in relation to the reference antenna.

[0017] A first object of the present invention is to provide a simpleyet accurate scheme for determining the direction of a transponder inrelation to a reference antenna. Another object of the present inventionis to provide a simple circuit for implementing the methods of thepresent invention. Still another object of the present invention is toenable the use of the disclosed systems and methods with backscatter tagreceivers that use limiting amplifiers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a diagram showing the geometry of a three antennadirection finding system in accordance with the present invention;

[0019]FIG. 2 is a schematic drawing of an exemplary direction findingsystem including a bi-static array of antennas;

[0020]FIG. 3 is a graph illustrating the difference channel signal fromthe bi-static array of antennas; and

[0021]FIG. 4 is a diagram showing another embodiment of the geometry ofa three antenna direction finding system in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0022] Referring now to the figures in which like reference numerals areused for like elements throughout, there is shown in FIG. 1, an antennaR transmits a signal to the area where communications with a transponderis desired. The signal is modulated and scattered by the transponder andis received by the antennas R, A and B. A first multichannel homodynereceiver can be connected to antenna R to provide the link forcommunications to and from the transponder (not shown). A microwavecombiner (element 201 in FIG. 2) is used to provide a second channel byforming the difference signal between antennas A and B. The differencesignal is delayed by 90° (i.e., ¼ wavelength).

[0023] A second multichannel homodyne receiver is attached to the secondchannel, and the polarity of the detected signal is compared to that ofthe first channel. If the polarities are the same, the transponder is tothe left of the antenna R. Conversely, if the polarities are opposite,the transponder is to the right of the antenna R. If there is not signaldetected in the second channel, the transponder is straight ahead. Anindication of the transponder location can be provided by an indicatoron the reader (not shown), which the system of the present invention isconnected to, to enable the operator to locate the transponder withwhich communications have been established. With the aid of theindicator, the operator can locate the transponder in a fashionanalogous to the pointing of a laser scanner to read bar codes. Thefirst channel may also be formed using the sum of signals from theantennas A and B instead of the antenna R. Many other uses of thelocation information are possible.

[0024] More particularly, the operation of the invention in accordancewith a first embodiment of the invention is analyzed below. As shown inFIG. 1, consider a transponder at a distance r from the three-elementantenna array in the direction θ. The spacing between the antennas is d.The distance r is assumed to be much greater than d, and the followinganalysis makes approximations based on that assumption. A signal istransmitted from the antenna R. The path length from the antenna R tothe transponder and returning to the antenna R is 2r, which correspondsto a phase change of 4πr/λ radians where Λ is the microwave wavelengthand π=3.14159265. The path length from the antenna R to the transponderand returning to the antenna B is shorter by the length a, wherein:

a=d sin(θ)

[0025] and the path length returning to the antenna A is longer bydistance a. The phase change corresponding to the length a is 2πa/λ. Thetime-varying nature of the signals, angular dependencies caused by thegains of the antennas at A, B, and R, the time variation of themodulation caused by the transponder and the variation in amplitude withthe distance are represented by the function f(r,t,θ). That functionneed not be expanded to complete the analysis to determine the directionto the transponder. The signals received by the three antennas can thenbe expressed as:

V _(R) =f(r,t,θ)cos(4πr/λ)

V _(A) =f(r,t,θ)cos(4πr/λ)+2πa/λ)

V _(B) =f(r,t,θ)cos(4πr/λ)−2πa/λ)

[0026] The sum of the signals of the antennas A and B is:

V _(A+B) =f(r,t,θ)cos(4πr/λ+2πa/λ)+f(r,t,θ)cos(4πr/λ−2πa/λ)

=f(r,t,θ)[cos(4πr/λ)cos(2πa/λ)−sin(4πr/λ)sin(2πa/λ)+cos(4πr/λ)cos(2πa/λ)+sin(4πr/λ)sin(2πa/λ)]

=2f(r,t,θ)cos(4πr/λ)cos(2πa/λ)

=2f(r,t,θ)cos(4πr/λ)cos(2πd sin(θ)/λ).

[0027] The difference between the signals of the antennas A and B is:

V _(A−B) =f(r,t,θ)cos(4πr/λ+πa/λ)−f(r,t,θ)cos(4πr/λ−2πa/π)

=f(r,t,θ)[cos(4πr/λ)cos(2πa/λ)−sin(4πr/λ)sin(2πa/λ)−cos(4πr/λ)cos(2πa/λ)−sin(4πr/λ)sin(2πa/π)]

=−2f(r,t,θ)sin(4πr/λ)sin(2πa/λ)

=−2f(r,t,θ)sin(4πr/λ)sin(2πd sin(θ)/λ).

[0028] Add a delay of 90° (π/2) to the path length of signal V_(A−B) toform signal V_(D):

V _(D) =V _(A−B) (delayed by 90 degrees)

=V _(A−B) (with 4πr/λ replaced by 4πr/λ+π/2)

=−2f(r,t,θ)sin(4πr/λ+π/2)sin(2πd sin(θ)/λ)

=−2f(r,t,θ)cos(4πr/λ)sin(2πd sin(θ)/λ).

[0029] The terms cos(4πr/λ) and sin(4πr/λ) result from the normalchanges in phase as the distance between the transponder and reader isvaried. The signal of the sum channel is always in phase with themonostatic channel (V_(R)), but the antenna pattern is narrower due tothe array effect. The signal of the difference channel which is delayedby 90° (V_(D)), is the same as the reference channel and the sum channelif the transponder is on the side of the array closer to the channelused as a positive reference, and is the inverse of the referencechannel and sum channel if the transponder is on the side opposite tothe channel used as positive reference for all values of r. When thetransponder is straight ahead, the difference channel output is zero.Thus, an electronic circuit that compares the signals can indicatewhether the transponder is to the right, to the left or straight ahead.An additional pair of antennas in the vertical plane can provide up/downindication as well, if desired. Comparison of the signals follows thedemodulation of the microwave signals. Because of the quadrature nulleffect, both in phase and quadrature signals should be used.

[0030] An embodiment of the required circuitry is shown in FIG. 2. Thesum and difference channels are formed from the microwave signalsreceived by the antennas A and B. The circuitry to implement theindication of direction consists of homodyne receivers 202, providing inphase, I, and quadrature, Q, outputs that recover the modulationproduced by the transponder in the normal fashion. Those signals areamplified by limiting amplifiers 204-210 that amplify the detectedsignals to logic levels. Those amplified I and Q signals of thedifference channel are compared to the output from the reference channel(not shown in FIG. 2 for clarity) or the sum channel. Those comparisonsare done with normal digital logic 212. Low pass filters (not shown) maybe used to eliminate noise and transients caused at signal transitionsand to provide an integration or smoothing effect for the outputs. Theindication or direction does not require decoding of the signals,synchronization with the clock of the data, or any other such digitaloperation. Thus, the indication of direction can be implemented by usingsimple circuitry.

[0031] The square of the term sin(2πd sin(θ)/λ) is plotted in FIG. 3 asa function of θ for various values of d/λ. The preferred value of d/λ isapproximately 0.5, although other antenna spacings can be used,depending on the desired physical width of the array and the width ofthe center null (which is used to provide transponder straight aheadindication). Thus, an operator of a reader can localize the transponderwith which he is communicating by repositioning the reader until anindication is provided that the desired transponder is straight ahead.Other uses of this information are also possible. Note that thistechnique functions with limiting amplifiers and no measurement of phaseangles nor computation by computer is needed.

[0032] In alternative embodiments of the invention, additional detailedinformation regarding the direction to a transponder is provided insteadof the general “right,” “left” or “center” indications of FIG. 2. Fouradditional methods to provide the actual angle θ are presented below.

[0033] In a first alternative method, electronically controlled phaseshifters 420 and 422 (FIG. 4) are added to the outputs of the antennas Aand B. The amount of phase shift is controlled to be identical and ofopposite polarity for the two channels. In that way, the “boresight”null is steered to the right or left by an amount controlled by thevalue of the inserted phase shift. The amount of steering can becalculated from the microwave wavelength λ, the antenna spacing d, andthe amount of inserted phase shift. The amount of inserted phase can beswept, and when the processing circuitry of FIG. 2 indicates thetransponder is at “center,” the value of θ is directly related to thevalue of the inserted phase. The value of θ can be derived by computer,microcontroller or by discrete circuitry. Alternatively, a “pointingcircuit” can be driven by the “left,” “right” and “center” outputs ofFIG. 2 to automatically adjust the inserted phase in a feedback loop topoint toward the transponder. The direction to the transponder is foundby the value of the inserted phase when the “pointing circuit” indicatesthat the transponder is at “center.” A value of θ can be found from thevalue of the inserted phase as above. That value can be used as desired,or other visual display can be provided to the operator, such as LEDdisplay, LCD display, voice synthesis, or the like.

[0034] In a second alternative method, if two antennas are used tocommunicate with a transponder, the relative phase can be determinedbetween each of the channels at the antenna outputs and a reference. Thedirection to the transponder can then be calculated knowing thedifference in phase between these two channels, the antenna spacing andthe microwave frequency. As shown in FIG. 1, a signal is transmitted toa transponder by the antenna R. The microwave signal from that antennais also used as the reference signal for the quadrature homodynereceivers at the antennas A and B. The phase measured at antennas A andB have an ambiguity of an unknown phase caused by the length of the pathto the transponder and back. Forming the mathematical difference inphase between the signals of the antennas A and B eliminates thatunknown amount and the result is caused by the length 2 a. The angle θcan be computed from the equation:

θ=arcsin(a/d)=arcsin(φλ/2πd)

[0035] where φ=φ_(A)−φ_(B)

[0036] The phases φ_(A) and φ_(B) are obtained by amplifying the outputsof the quadrature detector by a linear amplifier producing intermediatefrequency (IF) or baseband signals, filtering those signals to isolatethose due only to the modulation produced by the transponder, digitizingthe results with analog-to-digital (A/D) converters, and computing thephase angles with a computer or microcontroller. An angle is calculatedas the arctangent of the ratio of the quadrature signal to the in-phasesignal (e.g., Vq/Vi) at the outputs of the quadrature homodyne detector.

[0037] The third alternative method relates to an alternative to the useof linear amplifiers, phase meters and computers would be to vary thephase between the reference channels and the signal channels in acontrolled fashion, measure the locations of the quadrature nulls oneach channel, and subsequent calculation or other indication of theangle of arrival of the signal from the transponder. The direction tothe transponder can be found based on the differences in position of thequadrature nulls between the channels. This alternative method workswith limiting amplifiers and eliminates phase meters, but with the addedcost and complexity of requiring well controlled, low noise phaseshifters.

[0038] An implementation of that method is to use a phase shifter thatchanges phase linearly with voltage. That phase-changed signal is usedas the reference (or local oscillator (LO)) for the three quadraturehomodyne receivers. As the voltage is ramped, the signals in the threechannels are processed with a homodyne detector providing in phase andquadrature outputs. The detected signals are filtered by a bandpassfilter and amplified by a limiting amplifier. The signals in thechannels from the antennas A and B are each compared to the referencechannel. If the transponder is straight ahead (θ=0), then the signals inthe three channels will always be of the same polarity, even though eachchannel changes polarity due to the quadrature null effect (as either ror the inserted phase varies). If the transponder is to the right (θ ispositive), then there will be values of control voltage of the phaseshifter for which the signals will be of the same polarity, and valuesfor which the signals will be of the opposite polarity. The values ofthe inserted phase can be obtained by knowing the values of the controlvoltage. The differences in phase between the nulls is due to the phasecorresponding to the distance 2 a. Since d is known, a and thus θ can befound. A numerical value for θ can be calculated by the computer ormicrocontroller as described in the second alternative method.

[0039] The third method may use a technique for using the relativetimings of the occurrences of quadrature nulls for determining directionthat is analogous to using the timings of quadrature nulls fordetermining transponder motion as described in U.S. Pat. No. 5,510,795.As the phase of the LO reference signal is swept, quadrature nullconditions are observed at the outputs of the three homodyne receiversat different times. Since the inserted phase on the LO signalcorresponding to a 180 degree phase shift corresponds to the change inphase between quadrature null conditions on one channel, comparison ofthe timings of quadrature null conditions on the three channels providesa measurement of the relative phase between the channels. For example,if the phase of the LO signal is swept from 0 to 180 degrees, and thedifference in timing between quadrature nulls between antennas A and Bcorresponds to ¼ of the sweep time, the phase difference between thesignals received by antennas A and B is 45 degrees (180/4). Once thephase is known, the direction to the transponder is calculated by usingthe equation

θ=arcsin(a/d)

=arcsin(φλ/2πd)

[0040] This method does not require linear amplifiers, A/D converters,or phase meters. The added complexity of a phase shifter over theforegoing methods provides an actual value of the angle θ in addition tothe direction from the reader.

[0041] In a fourth alternative method of the present invention, thetransmit-receive properties and equations are the same as in theforegoing methods, but with the elimination of the 90° phase shifter ofFIG. 2. A homodyne receiver operates on the outputs of each of the threeantennas and each produces an in phase (I) and (Q) demodulated signal.These demodulated signals are combined in various ways to produce thesum and difference signals that are used to provide an indication of thedirection to the transponder. That method varies from the previousmethod in that the phases of the demodulated signals are not computed.

[0042] The foregoing methods can be used with antenna arrays of two ormore elements to determine the direction to a cooperative radar targetthat is producing modulated backscatter. While the foregoing descriptionused an array of three antennas, it should be appreciated that othercombinations of two or more antennas can also be used. The use oflimiting amplifiers is well known in the processing of signals inmodulated backscatter communications systems. Prior to the inventiondisclosed herein, there was no known way to also determine the directionto the transponder using the output from limiting amplifiers operatingon the outputs of homodyne quadrature detectors.

[0043] While this invention has been described in conjunction with thespecific embodiments outlined above, it is evident that manyalternatives, modifications and variations are apparent to those skilledin the art. Accordingly, the preferred embodiments of the invention asset forth above are intended to be illustrative and not limiting.Various changes may be made without departing from the spirit and scopeof the invention.

What is claimed is:
 1. A method for determining the direction to atransponder comprising: transmitting a first signal to an area wherecommunications with a transponder is desired; producing a second signalderived from the first signal; receiving the second signal via first,second, and third antennas; forming a difference signal from the secondsignal received via the first and second antennas; delaying thedifference signal; and comparing a first polarity of the delayeddifference signal with a second polarity of the second signal receivedvia the third antenna.
 2. The method of claim 1, further comprising:determining that the transponder is to the left of the third antennawhen the first and second polarities are the same.
 3. The method ofclaim 1, further comprising: determining that the transponder is to theright of the third antenna when the first and second polarities areopposite.
 4. The method of claim 1, further comprising: determining thatthe transponder is aligned with the third antenna when the delayeddifference signal is not detected by a receiver.
 5. The method of claim1, further comprising: receiving the second signal via fourth and fifthantennas; forming a second difference signal from the second signalreceived via the fourth and fifth antennas; delaying the seconddifference signal; and comparing a third polarity of the delayed seconddifference signal with the second polarity of the second signal receivedvia the third antenna.
 6. The method of claim 1, whereby delaying thedifference signals comprises delaying the difference signal by 90degrees.
 7. A method for determining the direction to a transpondercomprising: transmitting a first signal to an area where communicationswith a transponder is desired; producing a second signal desired fromthe first signal; receiving the second signal via a first and secondantenna forming a difference signal from the second signal received viathe first and second antennas; forming a third signal by adding thesecond signal received via the first antenna and the second signalreceive via the third antenna; delaying the difference signal; andcomparing a fist polarity of the delayed difference signal with a secondpolarity of the third signal.
 8. The method of claim 7, furthercomprising: determining that the transponder is to the left of a thirdantenna located in between and aligned with the first and secondantennas when the first and second polarities are the same.
 9. Themethod of claim 7, further comprising: determining that the transponderis to the right of a third antenna located in between and aligned withthe first and second antennas when the first and second polarities areopposite.
 10. The method of claim 7, further comprising: determiningthat the transponder is aligned with a third antenna located in betweenand aligned with the first and second antennas when the delayeddifference signal is not detected by a receiver.
 11. A system fordetermining the direction to a transponder comprising: a first antennafor transmitting a first signal to the transponder; second and thirdantennas for receiving a second signal from the transponder; means foradding the second signal received by the second antenna with the secondsignal received by the third antenna to produce a sum signal; means forsubtracting the second signal received by the second antenna from thesecond signal received by the third antenna to produce a differencesignal; means for delaying the difference signal; and a comparator forcomparing a first polarity of the difference signal with a secondpolarity of the sum signal.
 12. The system of claim 11, furthercomprising: a detector for detecting the delayed difference signal. 13.A system for determining the direction to a transponder comprising: afirst and a second antenna for receiving a signal from the transponder;means for adding the signal received by the first antenna with thesignal received by the second antenna to produce a sum signal; means forsubtracting the signal received by the first antenna from the signalreceived by the second antenna to produce a difference signal; and aprocessor for comparing the difference signal with the sum signal todetermine the direction to the transponder.